The present invention relates to a lighting device using an LED. More particularly, the present invention relates to an LED lighting device having an improved modulation depth which is calculated with the maximum value and the minimum value of an amount of instantaneous light emission in an AC-driven LED lighting device having an LED turned on and off by using an instantaneous AC voltage periodically varied with time.
A light emitting diode (LED), which is an electrophotic-conversion semiconductor device of emitting light when an electrical current flows therethrough, has been widely used as a backlight of a display device. As technology has been recently advanced, the electrophotic-conversion efficiency of the LED has been higher than those of glow lamps and fluorescent lamps, so that the LED has been widely used in general lighting applications.
As technologies for driving an LED lighting device, several AC-driven LED lighting devices which turn LEDs on and off by using an instantaneous AC voltage periodically varied with time have been developed.
Examples of the AC-driven LED lighting device are disclosed in U.S. Pat. Nos. 6,989,807 and 7,936,135, Japanese Patent No. 4581646, Japanese Patent Publication No. 2009-260505, Korean Patent publication Nos. 10-2011-0019213 and 10-2010-0136362 and Korean Patent No. 10-1110380 issued to an applicant of the present invention.
Hereinafter, problems of the prior art will be described with reference to FIGS. 1 and 2.
As shown in FIG. 1 (FIG. 15 of KR 10-1110380), an LED lighting device according to the related art includes an AC power source 1 for supplying AC voltage, a rectifier circuit 2 for rectifying the AC voltage, a plurality of LED blocks 11 to 14 connected in series to each other, bypass switches S11 to S14, each of which is connected in parallel to each of the LED blocks 11 to 14 to bypass current, a current source CS for limiting an amount of instantaneous AC current, and a controller 4.
At a specific time point of instantaneous AC voltage periodically varied with time, the circuit operates as follows:
1) The number of serially connected LED blocks 11 to 14 is controlled such that the serially connected LED blocks 11 to 14 are turned on with the instantaneous AC voltage at the specific time point of instantaneous AC voltage; and
2) The current source CS for limiting power current is controlled with a sine wave C sin in order to improve the power factor.
FIGS. 2a-2d (FIG. 18 of KR 10-1110380) are views showing currents flowing through each of the LED blocks 11 to 14 in the circuit of FIG. 1. In FIGS. 2a-2d, a current having a waveform 1AA flows through the LED block which is first turned on and a current having a waveform 4AA flows through the LED block which is lastly turned on.
It can be understood from current waveforms 1AA to 4AA flowing through the LED blocks that there exist sections where the current does not flow. Specifically, during the section where the current having the waveform 1AA is ‘0 (zero)’ (which is near the phase of 0° and 180° of the AC voltage, that is, portions corresponding to voltages less than the threshold voltage of one LED block), the currents having other current waveforms are also ‘0’ (zero).
That is, there exists a section (time) where light is not emitted. When the modulation depth is calculated with the maximum value and the minimum value of an amount of instantaneous light emission, the modulation depth becomes 100%.
Compared with 6% and 25%˜40% of the modulation depths of general glow and fluorescent lamps, the modulation depth of 100% is too high.